[flang] [clang] [flang][driver] Rename `flang-new` as `flang` (PR #74377)

[Andrzej Warzyński via cfe-commits]

banach-space wrote:

> Are you going to add an entry to the [Release Notes](https://github.com/llvm/llvm-project/blob/main/flang/docs/ReleaseNotes.md)?

Once this change lands, yes.

https://github.com/llvm/llvm-project/pull/74377